1. Effsys expand
May 18, 2016

2. Numerical study of the transitions of flow boiling in micro-channels
Qingming Liu, Björn Palm
Department of energy technology KTH

3. Contents
1 Introduction 2 Numerical methods 3 Boudary conditions 4 Results 5 Conclusions

4. Micro electronic chips
microchannels
MEMS(micro electronic and mechanic systems)
Micro electronic chips
Fuel cells
Micro heat exchangers

5. Contents
transitions

6. Flow boiling in 1,24 mm tube q= 15 kw/m2
Bubbly flow , confined bubbly flow , elongated bubbly flow, annular flow

7. Numerical methods
The Coupled Level Set and VOF method: higher accuracy on the gas-liquid interface curvature simulation than conventional VOF method A Non-equilibrium phase change model: higher accuracy on the interface temperature calculation than the equilibrium model

8. Validation of the interface curvature model (CLSVOF)
Single Taylor bubbles move in a channel

9. Validation of the interface temperature model
A gas bubble grows
in super heated liquids
1-D Stefan problem

10. Single bubble
Simulation on the flow and heat transfer characteristics of confined bubbles in micro-channels. ASME 10th International Conference on Nanochannels, Microchannels, and Minichannels, 2012

11. Effect of surface tension 𝜎𝑐𝑜𝑠𝜃= 𝜎 𝑔𝑠 − 𝜎 𝑙𝑠
(𝜎 =0.0039, , 𝑛 𝑚 ,
Inlet Re = 383, D=0.64 mm)

12. The motion of the bubble can enhancing micro convection by creating eddies between the bubble and wall.
Re= q= 30kw/m2

13. Results and Discussion Heat transfer
Wall temperature
fluid temperature
Re= q= 30kw/m2

14. Validation of the coalescence

15. Transition 1
A numerical study of bubble coalescence during flow boiling in micro-channels. APPLIED THERMAL ENGINEERING, Apr 2016
Effect of sub cooling on bubble coalescence. Micro and Nano flow conference London, Sept 2014

19. Three pairs of bubbles Saturated Sub-cooled Comparison with experiment
Owaihaib 2009

20. 𝐹𝑖𝑔. 9.𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
𝑐𝑜𝑛𝑡𝑜𝑢𝑟
𝑆𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑
S𝑢𝑏𝑐𝑜𝑜𝑙𝑒𝑑

21. Current work
Validation The causes of transition (Taylor bubble coalescence)

22. Adiabatic validation
Revellin, R. (2005). "Experimental two-phase fluid flow in microchannels."

23. A validation case
Fig.6.
Bubble shapes comparison with an experiment[32], the time span between two consecutive pictures is 0.2 ms . q=180 kw/m2, G=400 kg/m2,D=0.4 mm,Tsat= K

24. Future plan
The consequence of coalescence: 1 effect on heat transfer, 2 bubble terminal velocity 3 film thickness.